| Message ID | 20241009020149.23467-1-robbieko@synology.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | btrfs: reduce lock contention when eb cache miss for btree search | expand |
On Wed, Oct 9, 2024 at 3:09 AM robbieko <robbieko@synology.com> wrote: > > From: Robbie Ko <robbieko@synology.com> > > When crawling btree, if an eb cache miss occurs, we change to use > the eb read lock and release all previous locks to reduce lock contention. > > Because we have prepared the check parameters and the read lock > of eb we hold, we can ensure that no race will occur during the check > and cause unexpected errors. > > Signed-off-by: Robbie Ko <robbieko@synology.com> > --- > fs/btrfs/ctree.c | 88 ++++++++++++++++++++++++++++-------------------- > 1 file changed, 52 insertions(+), 36 deletions(-) > > diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c > index 0cc919d15b14..0efbe61497f3 100644 > --- a/fs/btrfs/ctree.c > +++ b/fs/btrfs/ctree.c > @@ -1515,12 +1515,12 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, > struct btrfs_tree_parent_check check = { 0 }; > u64 blocknr; > u64 gen; > - struct extent_buffer *tmp; > - int ret; > + struct extent_buffer *tmp = NULL; > + int ret, err; Please avoid declaring two (or more) variables in the same line. 1 per line is prefered for readability and be consistent with this function's code. > int parent_level; > - bool unlock_up; > + bool create = false; > + bool lock = false; > > - unlock_up = ((level + 1 < BTRFS_MAX_LEVEL) && p->locks[level + 1]); > blocknr = btrfs_node_blockptr(*eb_ret, slot); > gen = btrfs_node_ptr_generation(*eb_ret, slot); > parent_level = btrfs_header_level(*eb_ret); > @@ -1551,52 +1551,66 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, > */ > if (btrfs_verify_level_key(tmp, > parent_level - 1, &check.first_key, gen)) { > - free_extent_buffer(tmp); > - return -EUCLEAN; > + ret = -EUCLEAN; > + goto out; > } > *eb_ret = tmp; > - return 0; > + tmp = NULL; > + ret = 0; > + goto out; By setting tmp to NULL and jumping to the out label, we leak a reference on the tmp extent buffer. > } > > if (p->nowait) { > - free_extent_buffer(tmp); > - return -EAGAIN; > + ret = -EAGAIN; > + btrfs_release_path(p); To reduce the critical section sizes, please set ret to -EAGAIN after releasing the path. > + goto out; > } > > - if (unlock_up) > - btrfs_unlock_up_safe(p, level + 1); > + ret = -EAGAIN; > + btrfs_unlock_up_safe(p, level + 1); Same here, set ret after the unlocking. But why set ret to -EAGAIN? Here we know p->nowait is false. > + btrfs_tree_read_lock(tmp); > + lock = true; And here, with the same reasoning, set 'lock' to true before calling btrfs_tree_read_lock(). > + btrfs_release_path(p); > > /* now we're allowed to do a blocking uptodate check */ > - ret = btrfs_read_extent_buffer(tmp, &check); > - if (ret) { > - free_extent_buffer(tmp); > - btrfs_release_path(p); > - return ret; > + err = btrfs_read_extent_buffer(tmp, &check); > + if (err) { > + ret = err; > + goto out; Why do we need to have this 'err' variable? Why not use 'ret' and simplify? > } > - > - if (unlock_up) > - ret = -EAGAIN; > - > goto out; > } else if (p->nowait) { > - return -EAGAIN; > - } > - > - if (unlock_up) { > - btrfs_unlock_up_safe(p, level + 1); > ret = -EAGAIN; > - } else { > - ret = 0; > + btrfs_release_path(p); Same here, set 'ret' to -EAGAIN after releasing the path. > + goto out; > } > > + ret = -EAGAIN; > + btrfs_unlock_up_safe(p, level + 1); Same here. But why set ret to -EAGAIN? Here we know p->nowait is false. > + > if (p->reada != READA_NONE) > reada_for_search(fs_info, p, level, slot, key->objectid); > > - tmp = read_tree_block(fs_info, blocknr, &check); > + tmp = btrfs_find_create_tree_block(fs_info, blocknr, check.owner_root, check.level); > if (IS_ERR(tmp)) { > + ret = PTR_ERR(tmp); > + tmp = NULL; > btrfs_release_path(p); > - return PTR_ERR(tmp); > + goto out; > } > + create = true; > + > + btrfs_tree_read_lock(tmp); > + lock = true; Same here, set 'lock' to true before the call to btrfs_tree_read_lock(); > + btrfs_release_path(p); > + > + /* now we're allowed to do a blocking uptodate check */ Please capitalize the first word of a sentence and end the sentence with punctuation. This is the preferred style and we strive to do that for new comments or code that moves comments around. > + err = btrfs_read_extent_buffer(tmp, &check); This can block, so if p->nowait at this point we should instead exit with -EAGAIN and not call this function. > + if (err) { > + ret = err; > + goto out; > + } Same here, no need to use extra variable 'err', can just use 'ret'. > + > /* > * If the read above didn't mark this buffer up to date, > * it will never end up being up to date. Set ret to EIO now > @@ -1607,11 +1621,13 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, > ret = -EIO; > > out: > - if (ret == 0) { > - *eb_ret = tmp; > - } else { > - free_extent_buffer(tmp); > - btrfs_release_path(p); > + if (tmp) { > + if (lock) > + btrfs_tree_read_unlock(tmp); > + if (create && ret && ret != -EAGAIN) > + free_extent_buffer_stale(tmp); > + else > + free_extent_buffer(tmp); So in the success case we no longer set *eb_ret to tmp. Why? The callers expect that. Thanks. > } > > return ret; > @@ -2198,7 +2214,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root, > } > > err = read_block_for_search(root, p, &b, level, slot, key); > - if (err == -EAGAIN) > + if (err == -EAGAIN && !p->nowait) > goto again; > if (err) { > ret = err; > @@ -2325,7 +2341,7 @@ int btrfs_search_old_slot(struct btrfs_root *root, const struct btrfs_key *key, > } > > err = read_block_for_search(root, p, &b, level, slot, key); > - if (err == -EAGAIN) > + if (err == -EAGAIN && !p->nowait) > goto again; > if (err) { > ret = err; > -- > 2.17.1 > >
On Wed, Oct 9, 2024 at 3:18 PM Filipe Manana <fdmanana@kernel.org> wrote: > > On Wed, Oct 9, 2024 at 3:09 AM robbieko <robbieko@synology.com> wrote: > > > > From: Robbie Ko <robbieko@synology.com> > > > > When crawling btree, if an eb cache miss occurs, we change to use > > the eb read lock and release all previous locks to reduce lock contention. > > > > Because we have prepared the check parameters and the read lock > > of eb we hold, we can ensure that no race will occur during the check > > and cause unexpected errors. > > > > Signed-off-by: Robbie Ko <robbieko@synology.com> > > --- > > fs/btrfs/ctree.c | 88 ++++++++++++++++++++++++++++-------------------- > > 1 file changed, 52 insertions(+), 36 deletions(-) > > > > diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c > > index 0cc919d15b14..0efbe61497f3 100644 > > --- a/fs/btrfs/ctree.c > > +++ b/fs/btrfs/ctree.c > > @@ -1515,12 +1515,12 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, > > struct btrfs_tree_parent_check check = { 0 }; > > u64 blocknr; > > u64 gen; > > - struct extent_buffer *tmp; > > - int ret; > > + struct extent_buffer *tmp = NULL; > > + int ret, err; > > Please avoid declaring two (or more) variables in the same line. 1 per > line is prefered for readability and be consistent with this > function's code. > > > int parent_level; > > - bool unlock_up; > > + bool create = false; > > + bool lock = false; > > > > - unlock_up = ((level + 1 < BTRFS_MAX_LEVEL) && p->locks[level + 1]); > > blocknr = btrfs_node_blockptr(*eb_ret, slot); > > gen = btrfs_node_ptr_generation(*eb_ret, slot); > > parent_level = btrfs_header_level(*eb_ret); > > @@ -1551,52 +1551,66 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, > > */ > > if (btrfs_verify_level_key(tmp, > > parent_level - 1, &check.first_key, gen)) { > > - free_extent_buffer(tmp); > > - return -EUCLEAN; > > + ret = -EUCLEAN; > > + goto out; > > } > > *eb_ret = tmp; > > - return 0; > > + tmp = NULL; > > + ret = 0; > > + goto out; > > By setting tmp to NULL and jumping to the out label, we leak a > reference on the tmp extent buffer. Never mind about this one, it's the reference for the caller as we've set *eb_ret to tmp. > > > } > > > > if (p->nowait) { > > - free_extent_buffer(tmp); > > - return -EAGAIN; > > + ret = -EAGAIN; > > + btrfs_release_path(p); > > To reduce the critical section sizes, please set ret to -EAGAIN after > releasing the path. > > > + goto out; > > } > > > > - if (unlock_up) > > - btrfs_unlock_up_safe(p, level + 1); > > + ret = -EAGAIN; > > + btrfs_unlock_up_safe(p, level + 1); > > Same here, set ret after the unlocking. > > But why set ret to -EAGAIN? Here we know p->nowait is false. > > > + btrfs_tree_read_lock(tmp); > > + lock = true; > > And here, with the same reasoning, set 'lock' to true before calling > btrfs_tree_read_lock(). > > > + btrfs_release_path(p); > > > > /* now we're allowed to do a blocking uptodate check */ > > - ret = btrfs_read_extent_buffer(tmp, &check); > > - if (ret) { > > - free_extent_buffer(tmp); > > - btrfs_release_path(p); > > - return ret; > > + err = btrfs_read_extent_buffer(tmp, &check); > > + if (err) { > > + ret = err; > > + goto out; > > Why do we need to have this 'err' variable? > Why not use 'ret' and simplify? > > > } > > - > > - if (unlock_up) > > - ret = -EAGAIN; > > - > > goto out; > > } else if (p->nowait) { > > - return -EAGAIN; > > - } > > - > > - if (unlock_up) { > > - btrfs_unlock_up_safe(p, level + 1); > > ret = -EAGAIN; > > - } else { > > - ret = 0; > > + btrfs_release_path(p); > > Same here, set 'ret' to -EAGAIN after releasing the path. > > > + goto out; > > } > > > > + ret = -EAGAIN; > > + btrfs_unlock_up_safe(p, level + 1); > > Same here. > > But why set ret to -EAGAIN? Here we know p->nowait is false. > > > + > > if (p->reada != READA_NONE) > > reada_for_search(fs_info, p, level, slot, key->objectid); > > > > - tmp = read_tree_block(fs_info, blocknr, &check); > > + tmp = btrfs_find_create_tree_block(fs_info, blocknr, check.owner_root, check.level); > > if (IS_ERR(tmp)) { > > + ret = PTR_ERR(tmp); > > + tmp = NULL; > > btrfs_release_path(p); > > - return PTR_ERR(tmp); > > + goto out; > > } > > + create = true; > > + > > + btrfs_tree_read_lock(tmp); > > + lock = true; > > Same here, set 'lock' to true before the call to btrfs_tree_read_lock(); > > > + btrfs_release_path(p); > > + > > + /* now we're allowed to do a blocking uptodate check */ > > Please capitalize the first word of a sentence and end the sentence > with punctuation. > This is the preferred style and we strive to do that for new comments > or code that moves comments around. > > > + err = btrfs_read_extent_buffer(tmp, &check); > > This can block, so if p->nowait at this point we should instead exit > with -EAGAIN and not call this function. > > > + if (err) { > > + ret = err; > > + goto out; > > + } > > Same here, no need to use extra variable 'err', can just use 'ret'. > > > + > > /* > > * If the read above didn't mark this buffer up to date, > > * it will never end up being up to date. Set ret to EIO now > > @@ -1607,11 +1621,13 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, > > ret = -EIO; > > > > out: > > - if (ret == 0) { > > - *eb_ret = tmp; > > - } else { > > - free_extent_buffer(tmp); > > - btrfs_release_path(p); > > + if (tmp) { > > + if (lock) > > + btrfs_tree_read_unlock(tmp); > > + if (create && ret && ret != -EAGAIN) > > + free_extent_buffer_stale(tmp); > > + else > > + free_extent_buffer(tmp); > > So in the success case we no longer set *eb_ret to tmp. Why? The > callers expect that. > > Thanks. > > > } > > > > return ret; > > @@ -2198,7 +2214,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root, > > } > > > > err = read_block_for_search(root, p, &b, level, slot, key); > > - if (err == -EAGAIN) > > + if (err == -EAGAIN && !p->nowait) > > goto again; > > if (err) { > > ret = err; > > @@ -2325,7 +2341,7 @@ int btrfs_search_old_slot(struct btrfs_root *root, const struct btrfs_key *key, > > } > > > > err = read_block_for_search(root, p, &b, level, slot, key); > > - if (err == -EAGAIN) > > + if (err == -EAGAIN && !p->nowait) > > goto again; > > if (err) { > > ret = err; > > -- > > 2.17.1 > > > >
On Wed, Oct 9, 2024 at 3:18 PM Filipe Manana <fdmanana@kernel.org> wrote: > > On Wed, Oct 9, 2024 at 3:09 AM robbieko <robbieko@synology.com> wrote: > > > > From: Robbie Ko <robbieko@synology.com> > > > > When crawling btree, if an eb cache miss occurs, we change to use > > the eb read lock and release all previous locks to reduce lock contention. > > > > Because we have prepared the check parameters and the read lock > > of eb we hold, we can ensure that no race will occur during the check > > and cause unexpected errors. > > > > Signed-off-by: Robbie Ko <robbieko@synology.com> > > --- > > fs/btrfs/ctree.c | 88 ++++++++++++++++++++++++++++-------------------- > > 1 file changed, 52 insertions(+), 36 deletions(-) > > > > diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c > > index 0cc919d15b14..0efbe61497f3 100644 > > --- a/fs/btrfs/ctree.c > > +++ b/fs/btrfs/ctree.c > > @@ -1515,12 +1515,12 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, > > struct btrfs_tree_parent_check check = { 0 }; > > u64 blocknr; > > u64 gen; > > - struct extent_buffer *tmp; > > - int ret; > > + struct extent_buffer *tmp = NULL; > > + int ret, err; > > Please avoid declaring two (or more) variables in the same line. 1 per > line is prefered for readability and be consistent with this > function's code. > > > int parent_level; > > - bool unlock_up; > > + bool create = false; > > + bool lock = false; > > > > - unlock_up = ((level + 1 < BTRFS_MAX_LEVEL) && p->locks[level + 1]); > > blocknr = btrfs_node_blockptr(*eb_ret, slot); > > gen = btrfs_node_ptr_generation(*eb_ret, slot); > > parent_level = btrfs_header_level(*eb_ret); > > @@ -1551,52 +1551,66 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, > > */ > > if (btrfs_verify_level_key(tmp, > > parent_level - 1, &check.first_key, gen)) { > > - free_extent_buffer(tmp); > > - return -EUCLEAN; > > + ret = -EUCLEAN; > > + goto out; > > } > > *eb_ret = tmp; > > - return 0; > > + tmp = NULL; > > + ret = 0; > > + goto out; > > By setting tmp to NULL and jumping to the out label, we leak a > reference on the tmp extent buffer. > > > } > > > > if (p->nowait) { > > - free_extent_buffer(tmp); > > - return -EAGAIN; > > + ret = -EAGAIN; > > + btrfs_release_path(p); > > To reduce the critical section sizes, please set ret to -EAGAIN after > releasing the path. > > > + goto out; > > } > > > > - if (unlock_up) > > - btrfs_unlock_up_safe(p, level + 1); > > + ret = -EAGAIN; > > + btrfs_unlock_up_safe(p, level + 1); > > Same here, set ret after the unlocking. > > But why set ret to -EAGAIN? Here we know p->nowait is false. Nevermind this is because of the unconditional unlock now. > > > + btrfs_tree_read_lock(tmp); > > + lock = true; > > And here, with the same reasoning, set 'lock' to true before calling > btrfs_tree_read_lock(). > > > + btrfs_release_path(p); > > > > /* now we're allowed to do a blocking uptodate check */ > > - ret = btrfs_read_extent_buffer(tmp, &check); > > - if (ret) { > > - free_extent_buffer(tmp); > > - btrfs_release_path(p); > > - return ret; > > + err = btrfs_read_extent_buffer(tmp, &check); > > + if (err) { > > + ret = err; > > + goto out; > > Why do we need to have this 'err' variable? > Why not use 'ret' and simplify? > > > } > > - > > - if (unlock_up) > > - ret = -EAGAIN; > > - > > goto out; > > } else if (p->nowait) { > > - return -EAGAIN; > > - } > > - > > - if (unlock_up) { > > - btrfs_unlock_up_safe(p, level + 1); > > ret = -EAGAIN; > > - } else { > > - ret = 0; > > + btrfs_release_path(p); > > Same here, set 'ret' to -EAGAIN after releasing the path. > > > + goto out; > > } > > > > + ret = -EAGAIN; > > + btrfs_unlock_up_safe(p, level + 1); > > Same here. > > But why set ret to -EAGAIN? Here we know p->nowait is false. Nevermind this is because of the unconditional unlock now. In both cases we still don't need the 'err' variable. Just set 'ret' to -EAGAIN if btrfs_find_create_tree_block() below doesn't return an error and btrfs_read_extent_buffer() below and above don't return an error. > > > + > > if (p->reada != READA_NONE) > > reada_for_search(fs_info, p, level, slot, key->objectid); > > > > - tmp = read_tree_block(fs_info, blocknr, &check); > > + tmp = btrfs_find_create_tree_block(fs_info, blocknr, check.owner_root, check.level); > > if (IS_ERR(tmp)) { > > + ret = PTR_ERR(tmp); > > + tmp = NULL; > > btrfs_release_path(p); > > - return PTR_ERR(tmp); > > + goto out; > > } > > + create = true; > > + > > + btrfs_tree_read_lock(tmp); > > + lock = true; > > Same here, set 'lock' to true before the call to btrfs_tree_read_lock(); > > > + btrfs_release_path(p); > > + > > + /* now we're allowed to do a blocking uptodate check */ > > Please capitalize the first word of a sentence and end the sentence > with punctuation. > This is the preferred style and we strive to do that for new comments > or code that moves comments around. > > > + err = btrfs_read_extent_buffer(tmp, &check); > > This can block, so if p->nowait at this point we should instead exit > with -EAGAIN and not call this function. > > > + if (err) { > > + ret = err; > > + goto out; > > + } > > Same here, no need to use extra variable 'err', can just use 'ret'. > > > + > > /* > > * If the read above didn't mark this buffer up to date, > > * it will never end up being up to date. Set ret to EIO now > > @@ -1607,11 +1621,13 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, > > ret = -EIO; > > > > out: > > - if (ret == 0) { > > - *eb_ret = tmp; > > - } else { > > - free_extent_buffer(tmp); > > - btrfs_release_path(p); > > + if (tmp) { > > + if (lock) > > + btrfs_tree_read_unlock(tmp); > > + if (create && ret && ret != -EAGAIN) > > + free_extent_buffer_stale(tmp); > > + else > > + free_extent_buffer(tmp); > > So in the success case we no longer set *eb_ret to tmp. Why? The > callers expect that. Ok, it's because of the -EAGAIN due to having the unconditional unlock now. And btw, have you observed any case where this improved anything? Any benchmarks? I can't see how this can make anything better because this function is never called for a root node, and therefore level + 1 < BTRFS_MAX_LEVEL. But there is one case where this patch causes unnecessary path releases and makes the caller retry a search: when none of the levels above were locked. That's why we had the following expression before: unlock_up = ((level + 1 < BTRFS_MAX_LEVEL) && p->locks[level + 1]); So we wouldn't make the caller retry the search if upper levels aren't locked or p->skip_locking == 1. But now after this patch we make the caller retry the search in that case. Thanks. > > Thanks. > > > } > > > > return ret; > > @@ -2198,7 +2214,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root, > > } > > > > err = read_block_for_search(root, p, &b, level, slot, key); > > - if (err == -EAGAIN) > > + if (err == -EAGAIN && !p->nowait) > > goto again; > > if (err) { > > ret = err; > > @@ -2325,7 +2341,7 @@ int btrfs_search_old_slot(struct btrfs_root *root, const struct btrfs_key *key, > > } > > > > err = read_block_for_search(root, p, &b, level, slot, key); > > - if (err == -EAGAIN) > > + if (err == -EAGAIN && !p->nowait) > > goto again; > > if (err) { > > ret = err; > > -- > > 2.17.1 > > > >
Filipe Manana 於 2024/10/9 下午11:09 寫道: > On Wed, Oct 9, 2024 at 3:18 PM Filipe Manana <fdmanana@kernel.org> wrote: >> On Wed, Oct 9, 2024 at 3:09 AM robbieko <robbieko@synology.com> wrote: >>> From: Robbie Ko <robbieko@synology.com> >>> >>> When crawling btree, if an eb cache miss occurs, we change to use >>> the eb read lock and release all previous locks to reduce lock contention. >>> >>> Because we have prepared the check parameters and the read lock >>> of eb we hold, we can ensure that no race will occur during the check >>> and cause unexpected errors. >>> >>> Signed-off-by: Robbie Ko <robbieko@synology.com> >>> --- >>> fs/btrfs/ctree.c | 88 ++++++++++++++++++++++++++++-------------------- >>> 1 file changed, 52 insertions(+), 36 deletions(-) >>> >>> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c >>> index 0cc919d15b14..0efbe61497f3 100644 >>> --- a/fs/btrfs/ctree.c >>> +++ b/fs/btrfs/ctree.c >>> @@ -1515,12 +1515,12 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, >>> struct btrfs_tree_parent_check check = { 0 }; >>> u64 blocknr; >>> u64 gen; >>> - struct extent_buffer *tmp; >>> - int ret; >>> + struct extent_buffer *tmp = NULL; >>> + int ret, err; >> Please avoid declaring two (or more) variables in the same line. 1 per >> line is prefered for readability and be consistent with this >> function's code. Okay, I'll modify it. >> >>> int parent_level; >>> - bool unlock_up; >>> + bool create = false; >>> + bool lock = false; >>> >>> - unlock_up = ((level + 1 < BTRFS_MAX_LEVEL) && p->locks[level + 1]); >>> blocknr = btrfs_node_blockptr(*eb_ret, slot); >>> gen = btrfs_node_ptr_generation(*eb_ret, slot); >>> parent_level = btrfs_header_level(*eb_ret); >>> @@ -1551,52 +1551,66 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, >>> */ >>> if (btrfs_verify_level_key(tmp, >>> parent_level - 1, &check.first_key, gen)) { >>> - free_extent_buffer(tmp); >>> - return -EUCLEAN; >>> + ret = -EUCLEAN; >>> + goto out; >>> } >>> *eb_ret = tmp; >>> - return 0; >>> + tmp = NULL; >>> + ret = 0; >>> + goto out; >> By setting tmp to NULL and jumping to the out label, we leak a >> reference on the tmp extent buffer. We assign tmp to eb_ret, so we don't leak. >> >>> } >>> >>> if (p->nowait) { >>> - free_extent_buffer(tmp); >>> - return -EAGAIN; >>> + ret = -EAGAIN; >>> + btrfs_release_path(p); >> To reduce the critical section sizes, please set ret to -EAGAIN after >> releasing the path. Okay, I'll modify it. >> >>> + goto out; >>> } >>> >>> - if (unlock_up) >>> - btrfs_unlock_up_safe(p, level + 1); >>> + ret = -EAGAIN; >>> + btrfs_unlock_up_safe(p, level + 1); >> Same here, set ret after the unlocking. >> >> But why set ret to -EAGAIN? Here we know p->nowait is false. > Nevermind this is because of the unconditional unlock now. Yes, because we unlock unconditionally, the ret value of this function must be -EAGAIN. > >>> + btrfs_tree_read_lock(tmp); >>> + lock = true; >> And here, with the same reasoning, set 'lock' to true before calling >> btrfs_tree_read_lock(). >> >>> + btrfs_release_path(p); >>> >>> /* now we're allowed to do a blocking uptodate check */ >>> - ret = btrfs_read_extent_buffer(tmp, &check); >>> - if (ret) { >>> - free_extent_buffer(tmp); >>> - btrfs_release_path(p); >>> - return ret; >>> + err = btrfs_read_extent_buffer(tmp, &check); >>> + if (err) { >>> + ret = err; >>> + goto out; >> Why do we need to have this 'err' variable? >> Why not use 'ret' and simplify? >> >>> } >>> - >>> - if (unlock_up) >>> - ret = -EAGAIN; >>> - >>> goto out; >>> } else if (p->nowait) { >>> - return -EAGAIN; >>> - } >>> - >>> - if (unlock_up) { >>> - btrfs_unlock_up_safe(p, level + 1); >>> ret = -EAGAIN; >>> - } else { >>> - ret = 0; >>> + btrfs_release_path(p); >> Same here, set 'ret' to -EAGAIN after releasing the path. >> >>> + goto out; >>> } >>> >>> + ret = -EAGAIN; >>> + btrfs_unlock_up_safe(p, level + 1); >> Same here. >> >> But why set ret to -EAGAIN? Here we know p->nowait is false. > Nevermind this is because of the unconditional unlock now. > > In both cases we still don't need the 'err' variable. > Just set 'ret' to -EAGAIN if btrfs_find_create_tree_block() below > doesn't return an error and btrfs_read_extent_buffer() below and above > don't return an error. Indeed, we can only use ret value to receive the result of btrfs_find_create_tree_block or btrfs_read_extent_buffer. If the function returns correctly (return 0), we must change ret to -EAGAIN again and again, which is undoubtedly an increase in the function complexity. Therefore, I use the err variable. If other err occurs, the err can be set to ret, so that the caller can immediately stop and continue searching. > >>> + >>> if (p->reada != READA_NONE) >>> reada_for_search(fs_info, p, level, slot, key->objectid); >>> >>> - tmp = read_tree_block(fs_info, blocknr, &check); >>> + tmp = btrfs_find_create_tree_block(fs_info, blocknr, check.owner_root, check.level); >>> if (IS_ERR(tmp)) { >>> + ret = PTR_ERR(tmp); >>> + tmp = NULL; >>> btrfs_release_path(p); >>> - return PTR_ERR(tmp); >>> + goto out; >>> } >>> + create = true; >>> + >>> + btrfs_tree_read_lock(tmp); >>> + lock = true; >> Same here, set 'lock' to true before the call to btrfs_tree_read_lock(); >> >>> + btrfs_release_path(p); >>> + >>> + /* now we're allowed to do a blocking uptodate check */ >> Please capitalize the first word of a sentence and end the sentence >> with punctuation. >> This is the preferred style and we strive to do that for new comments >> or code that moves comments around. Okay, I'll modify it. >> >>> + err = btrfs_read_extent_buffer(tmp, &check); >> This can block, so if p->nowait at this point we should instead exit >> with -EAGAIN and not call this function. >> >>> + if (err) { >>> + ret = err; >>> + goto out; >>> + } >> Same here, no need to use extra variable 'err', can just use 'ret'. >> >>> + >>> /* >>> * If the read above didn't mark this buffer up to date, >>> * it will never end up being up to date. Set ret to EIO now >>> @@ -1607,11 +1621,13 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, >>> ret = -EIO; >>> >>> out: >>> - if (ret == 0) { >>> - *eb_ret = tmp; >>> - } else { >>> - free_extent_buffer(tmp); >>> - btrfs_release_path(p); >>> + if (tmp) { >>> + if (lock) >>> + btrfs_tree_read_unlock(tmp); >>> + if (create && ret && ret != -EAGAIN) >>> + free_extent_buffer_stale(tmp); >>> + else >>> + free_extent_buffer(tmp); >> So in the success case we no longer set *eb_ret to tmp. Why? The >> callers expect that. > Ok, it's because of the -EAGAIN due to having the unconditional unlock now. > > And btw, have you observed any case where this improved anything? Any > benchmarks? In the case of very high-speed 4k random write (cow), using nvme will improve. But I can't give it a clear data improvement, because there are still many different bottlenecks in the context of 4k random write, and other patches need to be used to achieve significant improvements. > > I can't see how this can make anything better because this function is > never called for a root node, and therefore level + 1 < > BTRFS_MAX_LEVEL. The main improvement point of this patch is that if a eb cache miss occurs in a leaf and needs to execute IO, our original approach is to release level 2~MAX, but this patch is changed to release level 1~MAX. We even Level 1 has also been released, so before releasing level 1, we changed to take the lock of level 0 to avoid race. This change reduces the lock contention of level 1. > > But there is one case where this patch causes unnecessary path > releases and makes the caller retry a search: > when none of the levels above were locked. That's why we had the > following expression before: > > unlock_up = ((level + 1 < BTRFS_MAX_LEVEL) && p->locks[level + 1]); > > So we wouldn't make the caller retry the search if upper levels aren't > locked or p->skip_locking == 1. > But now after this patch we make the caller retry the search in that case. Indeed, I missed something about skip_locking. In this case, there is no need to search again, I will correct it. Thank you for your review. > > Thanks. > >> Thanks. >> >>> } >>> >>> return ret; >>> @@ -2198,7 +2214,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root, >>> } >>> >>> err = read_block_for_search(root, p, &b, level, slot, key); >>> - if (err == -EAGAIN) >>> + if (err == -EAGAIN && !p->nowait) >>> goto again; >>> if (err) { >>> ret = err; >>> @@ -2325,7 +2341,7 @@ int btrfs_search_old_slot(struct btrfs_root *root, const struct btrfs_key *key, >>> } >>> >>> err = read_block_for_search(root, p, &b, level, slot, key); >>> - if (err == -EAGAIN) >>> + if (err == -EAGAIN && !p->nowait) >>> goto again; >>> if (err) { >>> ret = err; >>> -- >>> 2.17.1 >>> >>>
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c index 0cc919d15b14..0efbe61497f3 100644 --- a/fs/btrfs/ctree.c +++ b/fs/btrfs/ctree.c @@ -1515,12 +1515,12 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, struct btrfs_tree_parent_check check = { 0 }; u64 blocknr; u64 gen; - struct extent_buffer *tmp; - int ret; + struct extent_buffer *tmp = NULL; + int ret, err; int parent_level; - bool unlock_up; + bool create = false; + bool lock = false; - unlock_up = ((level + 1 < BTRFS_MAX_LEVEL) && p->locks[level + 1]); blocknr = btrfs_node_blockptr(*eb_ret, slot); gen = btrfs_node_ptr_generation(*eb_ret, slot); parent_level = btrfs_header_level(*eb_ret); @@ -1551,52 +1551,66 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, */ if (btrfs_verify_level_key(tmp, parent_level - 1, &check.first_key, gen)) { - free_extent_buffer(tmp); - return -EUCLEAN; + ret = -EUCLEAN; + goto out; } *eb_ret = tmp; - return 0; + tmp = NULL; + ret = 0; + goto out; } if (p->nowait) { - free_extent_buffer(tmp); - return -EAGAIN; + ret = -EAGAIN; + btrfs_release_path(p); + goto out; } - if (unlock_up) - btrfs_unlock_up_safe(p, level + 1); + ret = -EAGAIN; + btrfs_unlock_up_safe(p, level + 1); + btrfs_tree_read_lock(tmp); + lock = true; + btrfs_release_path(p); /* now we're allowed to do a blocking uptodate check */ - ret = btrfs_read_extent_buffer(tmp, &check); - if (ret) { - free_extent_buffer(tmp); - btrfs_release_path(p); - return ret; + err = btrfs_read_extent_buffer(tmp, &check); + if (err) { + ret = err; + goto out; } - - if (unlock_up) - ret = -EAGAIN; - goto out; } else if (p->nowait) { - return -EAGAIN; - } - - if (unlock_up) { - btrfs_unlock_up_safe(p, level + 1); ret = -EAGAIN; - } else { - ret = 0; + btrfs_release_path(p); + goto out; } + ret = -EAGAIN; + btrfs_unlock_up_safe(p, level + 1); + if (p->reada != READA_NONE) reada_for_search(fs_info, p, level, slot, key->objectid); - tmp = read_tree_block(fs_info, blocknr, &check); + tmp = btrfs_find_create_tree_block(fs_info, blocknr, check.owner_root, check.level); if (IS_ERR(tmp)) { + ret = PTR_ERR(tmp); + tmp = NULL; btrfs_release_path(p); - return PTR_ERR(tmp); + goto out; } + create = true; + + btrfs_tree_read_lock(tmp); + lock = true; + btrfs_release_path(p); + + /* now we're allowed to do a blocking uptodate check */ + err = btrfs_read_extent_buffer(tmp, &check); + if (err) { + ret = err; + goto out; + } + /* * If the read above didn't mark this buffer up to date, * it will never end up being up to date. Set ret to EIO now @@ -1607,11 +1621,13 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, ret = -EIO; out: - if (ret == 0) { - *eb_ret = tmp; - } else { - free_extent_buffer(tmp); - btrfs_release_path(p); + if (tmp) { + if (lock) + btrfs_tree_read_unlock(tmp); + if (create && ret && ret != -EAGAIN) + free_extent_buffer_stale(tmp); + else + free_extent_buffer(tmp); } return ret; @@ -2198,7 +2214,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root, } err = read_block_for_search(root, p, &b, level, slot, key); - if (err == -EAGAIN) + if (err == -EAGAIN && !p->nowait) goto again; if (err) { ret = err; @@ -2325,7 +2341,7 @@ int btrfs_search_old_slot(struct btrfs_root *root, const struct btrfs_key *key, } err = read_block_for_search(root, p, &b, level, slot, key); - if (err == -EAGAIN) + if (err == -EAGAIN && !p->nowait) goto again; if (err) { ret = err;